The present invention relates generally to hydraulic lift systems for multiple section or wide single section implements or the like, and more specifically, to a depth control system wherein the cylinders act in parallel through depth control valves to control individual sections or frame portions during field operation and wherein the cylinders operate essentially in series to permit the sections to move vertically in unison during raising and lowering to and from the transport position for turning, changing draft in the field, etc.
Wide implements such as field cultivators, chisel plows and air seeders typically include a main frame and outrigger frames pivotably connected to the main frame with separate hydraulic cylinders to control raising and lowering of the main and outrigger frames for proper depth control during field-working operations, for reducing hitch load to reduce spin-out and for movement to and from a transport position. Series cylinders have been utilized to assure that the different frame sections raise and lower in unison, but with a series system it is difficult or impossible to provide individual depth control of each section independently of the other sections. Series cylinder arrangements also require careful selection of cylinder size to assure operation in unison, and additional components are typically provided to rephase the cylinders upon full extension or retraction to compensate for leakage of hydraulic fluid.
Various depth control systems are available which utilize a hydraulic or electro-hydraulic control system to individually control the cylinder or cylinders on each separate frame section, such as shown in U.S. Pat. No. 4,600,060. A depth control valve for use with such a system is shown in U.S. Pat. No. 4,579,038. With such an arrangement as shown in the above-mentioned patents the cylinders are essentially operated in parallel during a fast raise/fast lower mode which results in uneven raising and lowering of the individual sections since the section having the least resistance will raise first and the section with the most weight will tend to lower first. Because the sections do not lift and lower in unison, ground-engagement and disengagement across the width of the implement will vary considerably unless the operator slows or stops the towing vehicle during the raise and lower modes at the end and start of a field-working operation.